1. Field of the Invention
The present invention relates to a knock detection apparatus for detecting a knock generated in an internal combustion engine and for controlling an ignition timing.
2. Description of the Related Art
It is generally known that ion is generated if fuel is burnt within a cylinder of an internal combustion engine. Therefore, if a probe to which a high tension voltage is applied it provided within the cylinder, it is possible to observe this ion in terms of the ionic current. Also, if the knock is generated in the internal combustion engine, since a vibratory component of the knock is superimposed on the ionic current, it is possible to detect the generation of the knock by extracting this vibratory component therefrom.
FIG. 13 is a circuit diagram showing a conventional knock detection apparatus using the ionic current. First of all, in this circuit, an ignition plug 1 is used as a detection probe for the ionic current. A high tension voltage (bias voltage) for detecting the ionic current by utilizing a secondary voltage of an ignition coil 2 is charged to a bias means 3. When the discharge for the ignition has been completed, the bias voltage charged during the discharge period is applied to an end of the plug 1 to detect the ionic current.
In this apparatus, a knock detection circuit 4 shapes a vibratory component, extracted from the ionic current, into a pulse form on the basis of a predetermined threshold value. A change of the number of pulses of the pulse form is calculated by an ECU 5. The ignition timing is adjusted by the result and the generation of the knock is suppressed.
In general, a peak value of the ionic current is changed in accordance with a kind of fuel or an operational condition of the internal combustion engine. However, there is a tendency that at a low rpm, the ionic current is small and at a higher rpm, is larger. The value thereof is in the range of several xcexcA to several hundreds of xcexcA.
FIG. 14 is a block diagram showing the knock detection circuit 4 shown in FIG. 13 in more detail. When the ionic current is fed by the high tension voltage applied by the bias means 3, the ionic current is distributed into a mask 9 and a BPF (band pass filter) 8 by a current distributing means 7 for extracting the vibratory component. The mask 9 is composed of a means for shaping the form of the ionic current by the predetermined threshold value to generate a pulse and a means for masking the pulse for a predetermined period of time for interrupting the noise by the ignition. The combustion/misfire may be judged in accordance with the pulse which will be referred to as a combustion pulse.
A window 10 starts an integration of the ionic current then the combustion pulse is turned on. When this integrated value reaches a predetermined value, a knock detection window is opened. The output is fixed so as not to generate the knock pulse until the integrated value reaches the predetermined value. Also, when the combustion pulse is turned off, the knock detection window is closed.
After the vibratory component of the knock has been extracted by the BPF 8, it is amplified by an amplifier 11 The vibratory component is shaped in accordance with the predetermined threshold value in a comparator portion 13 so that the knock pulse is generated The predetermined threshold value is set in a knock detection threshold setting portion.
FIG. 15 shows an operative shape example of each section of the circuit shown in FIG. 14. Also, FIG. 16 is an S/N graph of the number of the knock pulses upon the knock/non-knock.
The explanation has been made so far concerning the single cylinder. However, an actual engine is composed of a plurality of cylinders. Now, the engine having four cylinders will be exemplified. FIG. 17 is a block diagram showing a detection circuit for processing the ionic current of the four cylinders. The biasing means 3 used for an ionic current detection, the current distribution means 7 and the mask 9 for generating the combustion pulse are provided for each cylinder.
On the other hand, each knock detection circuit 4 (BPF 8, amplifier 11 and window 10) has one processing circuit commonly used for a pair of cylinders which is not adjacent to each other in ignition order, i.e., #1 and #4, and #2 and #3, in order to downsize the circuit. The ignition order of the engine is #1, #3, #4 and #2. Then, the final knock pulse output is the xe2x80x9cORxe2x80x9d of the outputs of the two processing circuits.
FIG. 18 shows an ignition signal (IB signal) for driving coils of the four cylinders, the ionic current and their vibratory component extraction waveforms. As shown in FIG. 18, when the ignition signal is turned on, the noise is generated in the ionic current. When the rpm of the engine is high or the ionic current generation period is long, the noise when the ignition signal is turned on in the knock detection period is overlapped with the knock detection period of the second former cylinder. Thus, there is a problem that the knock pulse is generated by the noise.
The knock pulse detected as mentioned before is transmitted to the ECU 5. In the ECU 5, a background level (knock judgement level) is calculated from the number of the knock pulses under the regular operational condition (when the knock is not generates. Then, it is judged that the knock is generated when the knock pulse exceeding the background level is generated, so that the ignition timing is changed in response to the knock strength in a direction in which the knock is not generated. When the knock is not generated, the ignition timing is likely to be gradually returned to the predetermined value to thereby perform the knock control.
However, if additives (such as K or Na) are mixed into the fuel, the ionic current is increased to a magnitude that is several time larger than that of the usual case even if the amount of the additives are small like several ppms. The ionic current has the same original frequency component. When the ionic current is increased, this frequency component becomes the same as the vibratory component upon the knock generation. In spite of the non-knock condition, the number of pulses is increased, so that the S/N ratio of the knock/non-knock disappears. There is a problem that the knock control is impossible. FIG. 19 shows the S/N ratio upon the containment of the additives measured under the same operational condition of the internal combustion engine as in that shown in FIG. 16.
The element relating to the increase/decrease of the ionic current is a time change of the internal combustion engine or the shape of the ignition plug in addition to the fuel characteristics. In these cases, the same problem might be also raised
In order to overcome the above-noted defect inherent in the prior art, an object of the present invention is to provide a knock detection apparatus for an internal combustion engine in which a threshold value of the knock detection is changed so that even if ionic current generation amount is changed due to the change of the fuel or the kinds of the plugs, it is possible to obtain a knock pulse S/N in accordance with a knock/non-knock.
Also, an object of the present invention is to provide a knock detection apparatus in which a knock pulse is not outputted by a noise upon turning on the ignition signal when means for extracting the vibratory components from the ionic current is commonly used in a pair of non-adjacent cylinders in the ignition order.
In order to achieve the above object, according to one aspect of the present invention, there is provided a knock detection apparatus for detecting a knock generated in an internal combustion engine and for controlling an ignition timing, in which a vibratory component superimposed on an ionic current generated by combustion of fuel is extracted, the component that is equal to or more than a knock detection threshold value is shaped in waveform into a pulse waveform, a knock strength is calculated through counting and calculation process of the number of pulses of the pulse waveform by an engine controlling means, an ignition timing is controlled on the basis of the knock strength, comprises a knock detection threshold value adjusting means for changing the knock detection threshold value on the basis of information of an ionic current.
According to another aspect of the present invention, there is provide the knock detection apparatus, wherein the knock detection threshold value adjusting means changes the knock detection threshold value on the basis of an integration value of the ionic current.
According to still another aspect of the present invention, there is provided a knock detection apparatus, wherein the knock detection threshold value adjusting means changes the knock detection threshold value on the basis of an integration value of the ionic current and a peak value of the ionic current.
According to a further aspect of the present invention, there is provided a knock detection apparatus, wherein the knock detection threshold value adjusting means changes the knock detection threshold value on the basis of an integration value of the ionic current, a peak value of the ionic current and a duty obtained by shaping the ionic current in waveform According to a still further aspect of the present invention, there is provided a knock detection apparatus, wherein the knock detection threshold value adjusting means smoothens a variation of the knock detection threshold value for every combustion cycle.
According to another aspect of the present invention, there is provided a knock detection apparatus, which further comprises a mask means for regarding a period during which said vibratory component of the ionic current is extracted as a period during which the combustion pulse is generated.